I decided to share this log as this trusty computer 计算机 been with me for 20 years and I just threw it away today.
I got myself a very new FX991. Well it is a 8 bit computer , with no ram no harddisk, with some register and with a display of 10+2 character.
What are the modification done to this machine ?
1) Overclock to 5X the speed. Tested with stopwatch, the slowest operation with this calculator is 110C52. The combinatorial function
C(n,k) is defined as n!/[k!(n-k)!], where n! is the factorial function.
Normal calculator can take up to 20s to find the answer. On my HP 11C it took over a minute.
After overclocking it took about 2s in non turbo mode and within 1sec in turbo mode.
2) added Rechargeable battery, these battery are charge using the solar panel allowing the calculator to be use without any light. I wounder why after so many years calculator still do not have this function. The rechargeable battery can last up to a week when new.
3) turbo, overvoltage overclocking. Crazy speed for bragging rights only.
Here I have a picture of the rechargeable battery switch. Have to on this to charge as well. The rechargeable battery is the one in blue above. They make the calculator FAT so that it would angle up for easy viewing , this open up the space available inside to add part required for this mod. Notice the wraping of plastic near the top, This is cause by overheating when I tried to charge the rechargeable directly under a lamp.
Here is the TURBO switch
Note the exam approval sticker at the back, The calculator with mod made it to A level, and all my U exam.
Next up is how overclocking and LCD contrast are adjusted, by changing the resistor on the RC (resistor capacitor circuit). Yes they do not have to invest in expensive crystal for clock control and it rely on an RC network, maybe they have a 555 inside. (555 is a clock control IC)
So after reading this article have you press 110C52 on your calculator to see just how long it take for it to calculate that. Got you!!!